A multiple structure grain is described, for example, in JP-A-60-143331, JP-A-62-196644 and JP-A-61-112142 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). In particular, JP-A-62-123445 describes a tabular multiple structure grain having an aspect ratio, namely, a ratio of the grain diameter to thickness of 1 or more. However, these patents relate mainly to a silver iodobromide emulsion and do not concern a high silver chloride emulsion which will be described in the present invention. The above-described patents either do not relate to a silver chloride emulsion having a {100} face as a main plane.
These silver iodobromide-type multiple structure grains are highly sensitive and excellent in pressure blackening durability, however, they are low in solubility as compared with silver chloride-type and cannot achieve high sensitivity and rapid processing property at the same time. The accumulation of iodide ions or bromide ions in the developer causes reduction in activity of the developer or inhibition of development. Also, the silver iodobromide emulsion is low in the fixing rate and has no rapid processing property.
There are quite a number of known techniques regarding tabular grains having a high silver chloride content. Examples of the tabular grains having a {111} face as a main plane are described in JP-B-64-8326 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-B-64-8325, JP-B-64-8324, JP-A-1-250943, JP-B-3-14328, JP-B-4-81782, JP-B-5-40298, JP-B-5-39459, JP-B-5-12696, JP-A-63-213836, JP-A-63-218938, JP-A-63-281149 and JP-A-62-218959.
Known techniques of the tabular grains having a {100} face as a main plane are described in JP-A-5-204073, JP-A-51-88017, JP-A-63-24238, U.S. Pat. No. 5,314,798 (corresponding to JP-A-6-347929 and EP 0534395A1), etc.
However, there is no disclosure on the multilayer-structure high silver chloride grain having a {100} face as a main plane comprising inside of the grain a high Br content layer.
The silver chloride has a crystal habit such that the {100} face is stable more than the {111} face and is suitable for achieving high sensitivity because the {100} face is known to be advantageous in view of dye adsorption or the like. However, if the silver chloride has a uniform structure, it is readily fogged at the chemical sensitization and since the grain has no mechanism therein to accelerate charge separation between the electrons generated at light absorption and the positive holes, inefficiency is present at the time of formation of a latent image.
Further, when the sensitivity is increased, the grain is extremely influenced the effect of pressure fogging. Accordingly, the grain as it is has failed to achieve concomitantly high sensitivity and pressure blackening durability.
Furthermore, if a layer having a maximum Br content is provided as the outermost layer of the grain, the initial fixing rate is retarded and thus, this is not the most preferred selection in achieving rapid fixing or low replenishment system of the fixing solution.